When drilling a well bore using a downhole drilling tool including a downhole motor, the drilling head can become snagged or stuck on the rock formation. In order to free the drilling head it is necessary to apply a very large torque to the drilling head, which is much larger than the torque available from the downhole motor. Therefore the torque must be applied to the drilling head from the surface through the drillstring.
If this is not possible, or the drilling head cannot be freed, then the downhole motor and drilling head have to be abandoned in the well bore, and the direction of the well bore diverted around the abandoned drilling head. This is extremely time consuming and expensive.
It is a problem that in a typical downhole motor, of the turbine type, there is no convenient way of transferring the torque of the drillstring to the drilling head, save for effectively jamming the mud motor or turbine shaft to the motor housing, which is connected to the drillstring. The housing of the turbine is mounted to the end of the drillstring, and therefore torque applied to the drillstring can be passed through the housing of the turbine. In one known method, metal balls or spheres are pumped down the centre of the drillstring to the turbine in the pressurized mud flow, which on reaching the turbine, lodge in cavities between the housing and the drive shaft of the turbine. The cavities are positioned and shaped such that when the housing is rotated in the drilling direction relative to the drilling head, the balls are forced into locking engagement with the housing and shaft, thus locking them together. The torque of the drillstring applied at the surface is therefore passed through the shaft to the drilling head and the required passage of fluid through the drillstring is not compromised.
A disadvantage of this method is that the torque from the drillstring is passed substantially through the whole length of the shaft of the turbine, which is primarily designed to transfer the torque generated by the turbine alone. The torque passed through the drillstring is significantly greater, and this can cause the shaft to fail, if the drilling head proves unmovable.
A further disadvantage of this method is that even if the drilling head is released from the rock formation, then the turbine and drilling head must be withdrawn to the surface to facilitate removal of the locking balls, before drilling can be resumed.